Oxidative Phosphorylation 2: Videos & Practice Problems

The Glycerol 3 Phosphate shuttle and the Malate-Aspartate shuttle are two mechanisms for transporting cytosolic NADH into the mitochondria. The Glycerol 3 Phosphate shuttle transfers electrons from NADH to FAD, producing FADH₂ and bypassing complex I, resulting in the production of 3 ATP. In contrast, the Malate-Aspartate shuttle transports NADH directly into the mitochondrial matrix without energy cost, yielding 5 ATP. This shuttle involves converting oxaloacetate to malate, which enters the matrix and is reoxidized to regenerate NADH. Aspartate is then formed and transported back to the cytosol, completing the cycle.

The Glycerol 3 Phosphate shuttle affects ATP yield by transferring electrons from cytosolic NADH to FAD, forming FADH₂. This process bypasses complex I of the electron transport chain, which normally pumps protons to generate a proton gradient. As a result, fewer protons are pumped, leading to the production of only 3 ATP per NADH molecule instead of the usual 5 ATP. This reduction occurs because FADH₂ contributes fewer protons to the gradient compared to NADH.

The Malate-Aspartate shuttle yields more ATP than the Glycerol 3 Phosphate shuttle because it transports NADH directly into the mitochondrial matrix without energy cost. This shuttle involves converting oxaloacetate to malate, which enters the matrix and is reoxidized to regenerate NADH. The NADH produced in the matrix can then enter the electron transport chain at complex I, leading to the production of 5 ATP. In contrast, the Glycerol 3 Phosphate shuttle transfers electrons to FAD, bypassing complex I and resulting in only 3 ATP.

Malate dehydrogenase plays a crucial role in the Malate-Aspartate shuttle by catalyzing the conversion of oxaloacetate to malate in the cytosol using NADH. This reaction reduces NADH to NAD⁺. Malate is then transported into the mitochondrial matrix, where it is reoxidized back to oxaloacetate, regenerating NADH. This NADH can then enter the electron transport chain at complex I, contributing to ATP production. The shuttle efficiently transfers electrons from cytosolic NADH into the mitochondrial matrix, facilitating oxidative phosphorylation.

The antiporter system in the Malate-Aspartate shuttle involves two key antiporters. The first antiporter transports malate into the mitochondrial matrix while simultaneously exporting α-ketoglutarate to the cytosol. Once inside the matrix, malate is reoxidized to oxaloacetate, regenerating NADH. The second antiporter moves aspartate out of the matrix and imports glutamate. Aspartate is then converted back to oxaloacetate in the cytosol, completing the cycle. This system ensures efficient electron transfer from cytosolic NADH to the mitochondrial matrix, facilitating ATP production.